"AIR" Like Noise In Rear Speakers

2000 FLHTC - I changed the front and rear speakers out on my bike with two matching sets of Sony speakers. I see under some of the radio sites that they have different part numbers for fronts and rears, but the speakers I took out were the same so I went ahead and put the same back in. Anyway, I have an air like sound in the rear speakers. You can hear it if you turn the rears all the way down (No noise at all in the fronts) - It doesn't increase with the volume but it is still there. Any thoughts??